In the case of such an exhaust system of a combustion engine the individual components of the exhaust system are exposed to a comparatively high thermal load, so that as a rule only metallic materials, in particular steel alloys are used in order to produce such components of an exhaust system. These components are mainly gas-conducting pipes and housings for exhaust system treatment devices such as for example silencers, particle filters, catalytic converters. Parts of these components, such as for example flanges, can be castings, preferentially cast steel components.
In addition to this, the hot combustion gases create an aggressive environment which in particular in conjunction with water or steam leads to a comparatively high corrosion risk for the components or cast components. There is a particularly high corrosion risk in the case of an SCR-system, with which a comparatively aggressive reduction agent is employed. Usually, in the case of an SCR-system, a watery urea solution is fed to the exhaust gas flow, wherein the urea reacts into ammonia through hydrolysis, which in an SCR-catalytic converter is used for the reduction of nitrogen oxides. Such an ammonia-containing atmosphere is comparatively aggressive and increases the corrosion hazard for the components and cast components involved. Corrosion-resistant steel alloys, i.e. stainless steel alloys, are for example austenitic. Accordingly, it is usual in the case of exhaust systems to use austenitic cast steel alloys for producing cast components, i.e. cast steel alloys with which cast components having an austenitic structure can be produced. Austenite cast steel however is comparatively expensive, which is in particular due to the nickel content of austenitic cast steel alloys.
However, ferritic cast steel alloys also exist. However, these are not all resistant to intercrystalline corrosion. Such an intercrystalline corrosion occurs for example at temperatures between 300° C. and 700° C. in conjunction with a corresponding aggressive environment. The intercrystalline corrosion is due to the known ferritic cast steel alloys becoming sensitized in the mentioned temperature range (300° C. and 700° C.) so that chromium carbides form which precipitate on the grain boundaries and withdraw the chromium from the surroundings near the grain boundary. However, chromium is decisive for the corrosion resistance of the alloy in conventional ferritic cast steel alloys. This sensitization takes place either during the operation of the exhaust system at the operating temperatures usually prevailing there, which lie in the mentioned temperature window, or even during the welding of the individual cast components or components, when these for example pass through said temperature window during cooling-down following the welding operation. A further consequence of the sensitization is the so-called grain decay, as a result of which the cohesion of the material in the structure is disturbed.
A further disadvantage of known ferritic cast steel alloys is the formation of comparatively large grains in the structure, which likewise has a negative effect on the resistance to intercrystalline corrosion. In addition, strength disadvantages are incurred.
Ferritic cast steel alloys are clearly more economical than austenitic cast steel alloys, so that there is great interest within the scope or large series production, particularly during the manufacture of exhaust system for combustion engine, to produce the cast components or component employed—as far as possible—from ferritic cast steel alloys. However, this is not possible with the currently available ferritic cast steel alloys in many cases for the mentioned reasons.
The present invention deals with the problem of stating an embodiment for a cast steel alloy, wherein the hazard of intercrystalline corrosion even at higher temperature loading is reduced, which for example occurs on the castings produced from it during joining, particularly during welding, and/or during operation of an exhaust system.